I thought the finale was okay. It was much longer than it had to be. At one point, I had paused it to get up when I thought I was near the ending, and it had 20 more minutes to go.
My wife who had made it much further through her reread of A Wrinkle in Time was surprised by how much material from the book was used in this last season. I'll comment more on that when I finish up my reread of the novel.
I am curious about the spin-off as they clearly end the series in the summer of 1989, so any new spin-off is going to be a 90s show. This should be interesting.
Doesn't have to be, a prequel is also a possibility and they left a pretty big story opening for it.
I would hate it if they retread the ground, built up a new villain from the Abyss or somewhere else and a new gang has to basically repeat the whole show. That would be a major disappointment given how well the DB's envisioned the show and how good it was written.
I figure we'll know soon enough once they and Netflix are ready to release some substantial information, which could well be a year or even two away and probably at a time when it's become an open secret when casting calls are going out and production crews are being put together.